The invention relates to a method for treating a suspension with one or more treatment liquids in order to maximize treatment efficiency and minimize heat loss. The invention is particularly applicable for the treatment of comminuted cellulosic fibrous suspensions (hereinafter "pulp"), particularly for the washing and bleaching of pulp.
In modern closed pulp mills the concentration of the pulp is maintained substantially constant throughout the entire treatment process, i.e. at a concentration of between about 7-15 percent. This is made possible by the utilization of continuous diffusers, which may be of the open type (such as shown in U.S. Pat. No. 3,372,087) or the pressure type (such as shown U.S. Pat. No. 4,368,628).
According to the present invention basic attributes of such diffusers are taken advantage of so as to provide a method of pulp treatment which has maximum efficiency, and with minimized heat loss. According to the method of the present invention, the pulp is caused to flow through a continuous diffuser type structure with a predetermined web thickness, preferably between about 50-500 mm, the flow path having first and second opposite sides. Treatment liquid is added to the pulp flowing in the flow path at a plurality of spaced predetermined points along the first side, while liquid displaced by the added treatment liquid is withdrawn from the flow path at substantially such predetermined points along the second side. The displaced liquid withdrawn from the first predetermined point consists essentially of original suspension liquid, while the liquid fraction of the pulp in the flow path past the last predetermined point preferably comprises mainly treatment liquid added at the last predetermined point. The concentration of the pulp is maintained substantially constant throughout treatment, preferably at a concentration between about 7-15 percent.
The pulp can be caused to flow in a number of series connected flow paths during treatment, with interconnections as desired in order to effect maximum efficiency of the treatment, depending upon the particular treatment being practiced (e.g. bleaching and/or washing), pressure, and temperature conditions, etc.
It is the primary object of the present invention to effect the continuous treatment of a suspension with one or more treatment liquids with maximum efficiency of treatment, and with minimal heat loss. These and other objects of the invention will become clear from an inspection of a detailed description of the invention, and from the appended claims.